The results of an experimental and theoretical study of the properties of a confocal unstable resonator used with a conventional CO2-N2-He laser operating at 10.6 μ are presented. Measurements were made of the near- and far-field intensity distribution of the aligned resonator for Fresnel numbers of 3.7, 6.1, and 7.1. Fundamental mode operation was achieved in all cases. Beam steering and mode degradation associated with mirror misalignment were also studied experimentally. Suitable geometric models have been developed to account for 1) the sensitivity of the resonator output coupling and center far-field intensity to variations in the resonator length and mirror curvatures, and 2) the sensitivity of the beam direction and power to angular misalignment of the mirrors.